linux-kernel - Re: [PATCH] mm: consider disabling readahead if there are signs of thrashing

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Message-ID: <87ikjywv16.fsf@linux.dev>
Date: Fri, 11 Jul 2025 09:29:25 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  Jan Kara <jack@...e.cz>,
  linux-mm@...ck.org,  linux-kernel@...r.kernel.org,  Liu Shixin
 <liushixin2@...wei.com>
Subject: Re: [PATCH] mm: consider disabling readahead if there are signs of
 thrashing

Matthew Wilcox <willy@...radead.org> writes:

> On Thu, Jul 10, 2025 at 12:52:32PM -0700, Roman Gushchin wrote:
>> We've noticed in production that under a very heavy memory pressure
>> the readahead behavior becomes unstable causing spikes in memory
>> pressure and CPU contention on zone locks.
>> 
>> The current mmap_miss heuristics considers minor pagefaults as a
>> good reason to decrease mmap_miss and conditionally start async
>> readahead. This creates a vicious cycle: asynchronous readahead
>> loads more pages, which in turn causes more minor pagefaults.
>
> Is the correct response to turn off faultaround, or would we be better
> off scaling it down (eg as low as 64k)?

I think at this point it better to turn it off entirely.

For scaling I wonder if we want to scale it depending on PSI numbers?

>
> I like the signal you're using; I think that makes a lot of sense.

Thanks!